1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus for reading out a radiation image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating light.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. There has been put into wide use as a computed radiography a radiation image recording and reproducing system using a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor). In the radiation image recording and reproducing system, a stimulable phosphor sheet is exposed to a radiation passing through an object such as a human body to have a radiation image information of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to two-dimensionally scan the stimulable phosphor sheet, thereby causing each part of the stimulable phosphor sheet exposed to the stimulating light beam to emit the stimulated emission, and the stimulated emission is photoelectrically detected, thereby obtaining an image signal (a radiation image signal) representing the radiation image information. In the radiation image information read-out apparatus employed in the radiation image recording and reproducing apparatus, it has been proposed to use a line light source which projects a line beam onto the stimulable phosphor sheet as a stimulating light source and to use a line sensor having an array of photoelectric convertor elements extending in the main scanning direction (the longitudinal direction of the line beam) as a means for photoelectrically reading out the stimulated emission. The line beam and the line sensor are moved relative to the stimulable phosphor sheet in the sub-scanning direction (the direction substantially perpendicular to the longitudinal direction of the line beam) by a scanning means. By the use of a line beam and a line sensor, the reading time is shortened, the overall size of the apparatus can be reduced and the cost can be reduced. See, for instance, Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540.
As a system using the stimulable phosphor sheet, there has been wide known an autoradiography detection system in which material labeled with radioactive labeling is administered to an organism, the organism or a part of the organism is taken as a sample, the sample is overlaid on a stimulable phosphor sheet for a predetermined time interval to have radiation image information of the object stored on the stimulable phosphor sheet, a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet, thereby causing each part of the stimulable phosphor sheet exposed to the stimulating light beam to emit the stimulated emission, and the stimulated emission is photoelectrically detected, thereby obtaining an image signal representing the radiation image of the sample. (See, for instance, Japanese Patent Publication Nos. 1(1989)-60784, 1(1989)-60782 and 4(1992)-3952.) In the autoradiography detection system, the stimulating light is caused to scan over the entire surface of the sample by moving the optical system in both the main scanning direction and the sub-scanning direction with a stage on which the sample is placed held stationary or by moving the optical system in the main scanning direction in which the stimulating light should scan the sample at a high speed and moving the stage in the sub-scanning direction in which the stimulating light may scan the sample at a relatively low speed.
The image signal obtained in each of the aforesaid systems is subjected to image processing such as gradation processing and/or frequency processing and a radiation image of the object is reproduced as a visible image for diagnosis on the basis of the processed radiation image signal on a recording medium such as a photographic film or a display such as a fine CRT. The doctor makes a diagnosis viewing the visible image or computer analysis is carried out on the basis of the visible image.
Further, there has been developed a microarray detection system in which known binding materials, e.g., hormones, tumor markers, enzymes, antibodies, antigens, abzymes, other proteins, nucleic acids, cDNAs, DNAs, mRNAs and the like, each of which can bond to a specific organism-derived material and properties of which such as the sequence, lengths, the composition and/or the like of bases are known are spotted by a spotter on a support such as a membrane filter to form a microarray of independent spots; an organism-derived material, e.g., hormone, tumor marker, enzyme, antibody, antigen, abzyme, other protein, nucleic acid, cDNA, DNA, mRNA or the like, which is sampled from an organic body (and is chemically processed or modified as desired) and labeled with radioactive labeling is hybridized with the known binding materials on the microarray; the microarray is kept in close contact with a stimulable phosphor sheet to expose the stimulable phosphor sheet in a pattern of distribution of the radioactive labeling on the microarray; a stimulating light beam such as a laser beam is caused to scan the stimulable phosphor sheet, thereby causing the stimulable phosphor sheet exposed to the stimulating light beam to emit the stimulated emission, and the stimulated emission is photoelectrically detected, thereby analyzing the organism-derived material sampled from the organic body. In the microarray detection system, as in the autoradiography detection system, the stimulating light beam may be caused to scan over the entire surface of the support by moving the optical system in both the main scanning direction and the sub-scanning direction with a stage on which the support is placed held stationary or by moving the optical system in the main scanning direction in which the stimulating light should scan the sample at a high speed and moving the stage in the sub-scanning direction in which the stimulating light may scan the sample at a relatively low speed. In the case of the microarray detection system, since the support is small in size, the stimulating light beam may be caused to scan the surface of the support by moving the support in the main scanning direction or in both the main scanning direction and the sub-scanning direction.
In the systems where a stimulable phosphor sheet storing thereon radiation energy is two-dimensionally scanned by a stimulating light beam and stimulated emission emitted from the stimulable phosphor sheet upon stimulation by the stimulating light beam is detected, it has been conceived that since the intensity of the stimulated emission is proportional to the power of the stimulating light beam, the stimulated emission can be detected at a higher sensitivity as the power of the stimulating light beam is increased.
However, an experiment of these inventors has revealed that the sensitivity of detection cannot be increased with increase of the stimulating light power beyond a predetermined limit but deteriorates when the stimulating light power is increased beyond the predetermined limit. Further, when the read-out pitches in the sub-scanning direction are narrowed in order to meet demand for a higher resolution, the deterioration of the sensitivity of detection becomes more significant.
When the stimulating light beam scans the stimulable phosphor sheet along a main scanning line, the stimulating light beam can stimulate the stimulable phosphor sheet not only along the main scanning line but also along a main scanning line or main scanning lines following due to scattering of the stimulating light in the stimulable phosphor layer, which can results in so-called lost-reading (a part of the stimulable phosphor sheet which has been exposed to the stimulating light and emitted radiation energy is scanned by the stimulating light) and significant deterioration in the sensitivity of detection. This tendency is more significant when the read-out pitches in the sub-scanning direction are narrowed in order to meet demand for a higher resolution. Further, the experiment has revealed that the phenomenon is suppressed when the beam diameter of the stimulating light is smaller than a predetermined value since the phenomenon is due to scattering of the stimulating light in the stimulable phosphor layer.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image read-out method and apparatus for reading out a radiation image stored on a stimulable phosphor sheet by scanning the stimulable phosphor sheet with stimulating light in a main scanning direction and a sub-scanning direction and detecting stimulated emission emitted from the stimulable phosphor sheet upon stimulation by the stimulating light in which the radiation image can be read-out at a high sensitivity and a high resolution.
In accordance with a first aspect of the present invention, there is provided a radiation image read-out method in which a stimulable phosphor sheet stored thereon a radiation image is two-dimensionally scanned by a stimulating light beam by projecting the light beam along a main scanning line extending in a main scanning direction and moving one of the stimulable phosphor sheet and the stimulating light beam in a sub-scanning direction relatively to the other so that the stimulable phosphor sheet is exposed to the stimulating light beam along a plurality of main scanning lines arranged in the sub-scanning direction at a predetermined pitch and stimulated emission emitted from the part of the stimulable phosphor sheet exposed to the stimulating light beam is photoelectrically converted to an electric image signal by a photoelectric convertor means, wherein the improvement comprises the step of
controlling the power of the stimulating light beam depending on the pitch at which the main scanning lines are arranged in the sub-scanning direction.
The stimulable phosphor sheet may be two-dimensionally scanned by the stimulating light beam, by projecting the light beam along the main scanning line extending in the main scanning direction and moving one of the stimulable phosphor sheet and the stimulating light beam in the sub-scanning direction relatively to the other, in any way. For example, a spot of the stimulating light beam may be caused to two-dimensionally scan the stimulable phosphor sheet by moving the spot of the stimulating light beam in both the main scanning direction and the sub-scanning direction with the stimulable phosphor sheet kept stationary or by moving the spot of the stimulating light beam in one of the main scanning direction and the sub-scanning direction with the stimulable phosphor sheet moved in the other of the directions. Further, the stimulable phosphor sheet may be moved in both the main scanning direction and the sub-scanning direction with the spot of the stimulating light beam kept stationary. Further, a stimulating light source which projects stimulating light on the stimulable phosphor sheet in a pattern of a line beam extending in the main scanning direction may be employed and one of the line beam and the stimulable phosphor sheet may be moved in the sub-scanning direction.
The photoelectric convertor means may be any means so long as it can convert the stimulated emission to an electric signal. For example, a photomultiplier, a CCD sensor or a line sensor comprising a plurality of photoelectric convertor elements arranged in a row may be used.
xe2x80x9cThe power of the stimulating light beamxe2x80x9d means the amount of energy of the stimulating light projected onto the stimulable phosphor sheet and xe2x80x9ccontrolling the power of the stimulating light beamxe2x80x9d means to control the amount of energy of the stimulating light projected onto the stimulable phosphor sheet by controlling the intensity of the stimulating light beam and/or one of the scanning speed in the main scanning direction and that in the sub-scanning direction.
xe2x80x9cControlling the power of the stimulating light beam depending on the pitch at which the main scanning lines are arranged in the sub-scanning directionxe2x80x9d (this pitch will be sometimes referred to as xe2x80x9cthe sub-scanning pitchxe2x80x9d hereinbelow) means to control the power of the stimulating light beam, for instance, in the following way. The output P of the photoelectric convertor means versus the stimulating light power is as shown in FIG. 2, and in any of sub-scanning pitches of 25 xcexcm, 500 xcexcm and 100 xcexcm, though increasing with increase of the stimulating light power up to a certain value, the output P of the photoelectric convertor means is rather reduced with increase of stimulating light power beyond the certain value due to the influence of radiation energy release caused by scattering of the stimulating light. Accordingly, the stimulating light power should be controlled to the value where an optimal output of the photoelectric convertor means can be obtained for each sub-scanning pitch, e.g., to I25, I50 or I100 where the output of the photoelectric convertor means is maximized for each sub-scanning pitch.
For example, the power I of the stimulating light beam may be in the range of a half of I0 represented by the following formula (1) to twice the same.
I0=1/"sgr"1xc2x7ln(1+"sgr"2/"sgr"1)xe2x80x83xe2x80x83(1)
wherein "sgr"1 is an absorption factor based on the retention of radiation energy represented by the ratio of an output signal of the photoelectric convertor means when the stimulating light is projected onto a first part of a stimulable phosphor sheet storing therein radiation energy to that when the stimulating light is projected onto a second part the stimulable phosphor sheet deviated from the first part in the sub-scanning direction, "sgr"2 is an absorption factor based on the read-out efficiency represented by the ratio of an output signal of the photoelectric convertor means when the stimulating light is projected onto a stimulable phosphor sheet storing therein radiation energy to the power of the stimulating light.
Formula (1) is derived in the following manner. That is, the relation between the output P of the photoelectric convertor means and the stimulating light power I shown in FIG. 2 can be theoretically represented by the following formula (2).
P=N0exe2x88x92"sgr"1xc2x7I(1xe2x88x92exe2x88x92"sgr"2xc2x7I)xe2x80x83xe2x80x83(2)
wherein N0 represents the amount of radiation energy stored in the stimulable phosphor sheet, exe2x88x92"sgr"1xc2x7I represents the retention of radiation energy on the following main scanning line when stimulating light is projected onto the stimulable phosphor sheet along a main scanning line and (1xe2x88x92exe2x88x92"sgr"2xc2x7I) represents the read-out efficiency represented by the ratio of an output signal of the photoelectric convertor means when the stimulating light is projected onto a stimulable phosphor sheet to the power of the stimulating light at that time. Accordingly, the retention of radiation energy exe2x88x92"sgr"1xc2x7I is 1 and the read-out efficiency (1xe2x88x92exe2x88x92"sgr"2xc2x7I) is 0 when the power I of the stimulating light is 0.
The power I0 of the stimulating light at which the output signal is maximized for each sub-scanning pitch is obtained when dP/dI=0 and is as represented by formula (1).
By substituting a value of "sgr"1 dependent on the read-out pitch and a value of "sgr"2 independent of the read-out pitch in formula (1), an optimal range of the power of the stimulating light for each pitch can be determined.
In accordance with a second aspect of the present invention, there is provided a radiation image read-out apparatus comprising stimulating light projecting means which two-dimensionally scans a stimulable phosphor sheet stored thereon a radiation image with a stimulating light beam by projecting the light beam along a main scanning line extending in a main scanning direction and moving one of the stimulable phosphor sheet and the stimulating light beam in a sub-scanning direction relatively to the other so that the stimulable phosphor sheet is exposed to the stimulating light beam along a plurality of main scanning lines arranged in the sub-scanning direction at a predetermined pitch and a photoelectric convertor means which detects stimulated emission emitted from the part of the stimulable phosphor sheet exposed to the stimulating light beam and photoelectrically converts the stimulated emission to an electric image signal, wherein the improvement comprises
a stimulating light power controlling means which controls the power of the stimulating light beam depending on the pitch at which the main scanning lines are arranged in the sub-scanning direction.
For example, the stimulating light power controlling means may control the power I of the stimulating light beam in the range of a half of I0 represented by the following formula (1) to twice the same.
I0=1/"sgr"1xc2x7ln(1+"sgr"2/"sgr"1)xe2x80x83xe2x80x83(1)
wherein "sgr"1 is the absorption factor based on the retention of radiation energy represented by the ratio of an output signal of the photoelectric convertor means when the stimulating light is projected onto a first part of a stimulable phosphor sheet storing therein radiation energy to that when the stimulating light is projected onto a second part the stimulable phosphor sheet deviated from the first part in the sub-scanning direction, "sgr"2 is the absorption factor based on the read-out efficiency represented by the ratio of an output signal of the photoelectric convertor means when the stimulating light is projected onto a stimulable phosphor sheet storing therein radiation energy to the power of the stimulating light.
The sub-scanning pitch may be, for instance, not larger than 50 xcexcm or not larger than 25 xcexcm.
In accordance with the present invention, since the stimulating light power is controlled depending on the sub-scanning pitch, the stimulating light power can be at such a value that can provide an optimal sensitivity at the sub-scanning pitch, whereby the radiation image can be read out at a high sensitivity and at a high resolution. Further, since the stimulating light power which will provide a highest sensitivity reduces with increase in resolution, the read-out speed can be significantly increased when the stimulating light power is reduced by increasing read-out speed (the stimulating light power can be reduced by increasing the read-out speed as described above) as the resolution increases.
When the power I of the stimulating light beam is controlled in the range of a half of I0 represented by the aforesaid formula (1) to twice the same, an optimal range of the stimulating light power depending on the sub-scanning pitch can be easily obtained by only calculating values of "sgr"1 and "sgr"2 by simple measurement.